Electricity is typically generated by producing steam which drives a turbine to operate an electrical generator. The steam can be provided by a variety of methods, including the radioactive decay of uranium in nuclear reactors, the combustion of coal, the burning of oil, or other means. One such other means is the combustion of non-traditional fuels such as wood, paper, trash, and fossil fuels not suitable for pulverized coal boilers or oil boilers, such as low quality coal or anthracite. Such fuels may be combusted to produce steam in a boiler called a circulating fluidized bed.
Circulating fluidized bed boilers have several advantages over pulverized coal boilers. Coal is not readily available everywhere. More particularly, coal that combusts with low noxious emissions is not readily available everywhere and is sometimes prohibitively expensive. Emissions are closely regulated by environmental regulations directed toward sulfur and nitrogen oxides as well as ash content. Circulating fluidized bed systems produce ash, but tend to be low in sulfur and nitrogen oxide emissions. They can be used to combust coal, and also low bituminous coal, anthracite, lignite, paper, wood and even trash.
A typical circulating fluidized bed system includes a combustor, a solids recycle loop or "cyclone," and an ash collector. The combustor functions to combust fuel; the solids recycle loop recirculates uncombusted solid fuel exiting the combustor back to the combustor for combustion; and the ash collector functions to collect ash.
The combustor includes a bed of sand or other inert granular material. In operation, the bed is "fluidized" at a high temperature sufficient to spontaneously combust fuel entering the combustor.
The combustor normally includes one or more "diffusers" which operate to diffuse the sand bed. The diffusers are turbines or propellers mounted on the end of a rotating staff in a port in the wall of the combustor. The shaft is extendable and retractable from the ports in the combustor wall. In the extended position the shaft angles downward and toward the center of the combustor bed to allow the propeller or turbine to diffuse the sand or other granular material. In the retracted position the shaft along with the attached propeller or turbine is within the port in the wall.
The ports housing the diffusers are prone to erosion by the impact of abrasive ash and other particulates within the boiler. In the prior art, the ports are flush with the refractory hood by means of trowel, ramming, and/or other manual methods with the refractory of the combustor interior surface. High velocity ash and other particles colliding with the corner of the refractory wall where it meets the diffuser port continually erode the refractory to the point where the shielding and insulating function is diminished. If the diffuser is mounted within a metal tube positioned within the port, as is often the case, the metal tube itself is eroded and the boundary between the metal tube and refractory is the site of additional erosion.